The major aim of this project is to reexamine the role of endothelium in the initiation of atherosclerotic lesion development in experimental animals. It has been shown that endothelial cell loss does not occur prior to early fatty streak development and yet there is a body of evidence linking endothelial cell loss and platelet mitogens to atherogenesis. To try to resolve the role of endothelial integrity in atherosclerosis, we will determine whether loss of endothelium does occur even though the vessel has the appearance of an intact endothelium and whether platelets and more importantly platelet products can be present without any obvious morphological loss of endothelium. Particular studies will determine whether luminal smooth cells can be mistaken for endothelial cells and whether they are present in arteries thought to be covered by an intact endothelium. Furthermore, studies will investigate why endothelial cells cannot repair large denuding injuries thereby allowing smooth muscle cells to form a pseudoendothelium. Such events would explain the supposed intact endothelium over intimal lesions which contain proliferating smooth muscle cells. We will model this process in vitro and then determine whether cessation of growth is due to an excessive number of doublings or to other factors such as the presence of endothelial growth inhibitory proteins. To further explore the possibility of arterial lesion formation without loss of endothelium, experiments will determine whether platelet proteins and/or label released at one site of endothelial injury can interact with the vessel wall at a distant site where no endothelial injury is present. Uptake of platelet products will be assessed in the cholesterol-fed rabbit using indium-111-labeled platelets and with antibody to platelet factor 4. The above studies are part of an ongoing theme to investigate what role endothelium plays in the initiation of arterial lesion formation.